Conventional directional antennas, such as a Traffic Alert Collision Avoidance System (TCAS) directional antenna, are used in commercial, military, and private aircraft applications to detect and warn of potential collisions with other air traffic. Typically, such directional antenna systems include some apparatus for detecting failures in the antenna. For example, conventional directional antennas may employ internal test circuitry that is used to detect failures in the antenna and to ensure that the cables are correctly attached.
FIG. 1 shows an example of a layout of such a circuit. The schematic of this circuitry is shown in FIG. 2. The function of the test circuitry is centered on four resistors that can be sensed by a computer unit. Each resistor has a different resistance value so that each channel of the directional antenna has a unique DC characteristic for ease in detection and isolation of failure. However, these resistors are isolated by the use of capacitors so that each channel can be sensed independently. As such, when one of the capacitors or solder joints fails as an open circuit, detection by a DC continuity check becomes difficult. This results in an undetected failure mode of the antenna. The result of this undetected capacitor or solder joint failure is radiation pattern degradation. Such degradation can be the cause of traffic being displayed in the wrong location on the pilot's display. While the bearing of an intruder aircraft is not used to determine any resolution advisory information, the pilot's confidence in a system possibly displaying incorrect information may be eroded.
Other solutions for antenna failure detection have made use of detecting the voltage standing wave ratio (VSWR) of the antenna ports and characterizing what VSWR values relate to a failed capacitor (open circuit). Unfortunately, VSWR detectors typically have large indeterminate zones where they are not reliable in detecting an out of specification VSWR. In addition, as VSWR detectors are typically located in a computer unit (which is typically further isolated from the antenna by a cable with 2–3 dB attenuation), even more uncertainty is added.